1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a multi-domain liquid crystal display device to having an improved aperture ratio as well as reliable image quality.
2. Discussion of the Related Art
Lately, a liquid crystal display device is mainly used as a flat panel display device of low power consumption. The liquid crystal display device includes a thin film transistor array substrate, a color filter substrate bonded to the thin film transistor array substrate to leave a prescribed interval, and a liquid crystal layer between the thin film transistor array substrate and the color filter substrate.
A plurality of pixels are arranged matrix-like on the thin film transistor array substrate. And, a thin film transistor, a pixel electrode, and a capacitor are formed in each unit pixel. A common electrode for applying an electric field to the liquid crystal layer together with the pixel electrodes, RGB color filters implementing a real color, and a black matrix are formed on the color filter substrate.
Meanwhile, an alignment layer is formed on respective confronting surfaces of the thin film transistor array substrate and the color filter substrate. Rubbing is carried out on the alignment layer to align the liquid crystal layer in a uniform direction. In this case, liquid crystals rotate by dielectric anisotropy if the electric field is applied between the corresponding pixel electrode of each of the unit pixels of the thin film transistor array substrate and the common electrode on the color filter substrate, whereby light passes through the unit pixel or is cut off to display a character or image. Yet, the above-explained TN (twisted nematic) mode liquid crystal display device provides a narrow viewing angle. This is attributed to refractive anisotropy of liquid crystal molecules. In case of TN mode, light transmission for a horizontal viewing angle is symmetrically distributed but shows asymmetric distribution for a vertical viewing angle. Hence, a range of image inversion is generated from the vertical viewing angle so as to decrease the corresponding viewing angle.
In order to overcome such a viewing angle problem, a multi-domain liquid crystal display device is proposed to compensate the viewing angle, in which a pixel is divided into at least two domains, and a main viewing angle direction of each domain is differentiated like TDTN (two domain TN) or DDTN (domain divided TN). A fabrication method of a multi-domain liquid crystal display device includes photolithography and rubbing.
FIGS. 1A to 2B are cross-sectional views for implementing multi-domain. FIGS. 1A to 1C illustrate a method of implementing multi-domain using a rubbing direction, and FIGS. 2A and 2B show a method of implementing multi-domain using photo alignment.
Referring to FIG. 1A, an alignment layer 3 is coated on a substrate 1 on which such a plurality of patterns (not shown in the drawing) as thin film transistors and color filters. Rubbing is carried out on the alignment layer 3 in one direction using a rotating rubbing roll 5. In this case, rubbing is a process for determining an initial alignment direction of liquid crystal molecules.
Referring to FIG. 1B, a photoresist is coated on the alignment layer 3 rubbed in one direction. A photoresist pattern 7 is then formed by photolithography to expose a portion of the alignment layer 3.
Referring to FIG. 1C, the rubbing roll 5 is made to rotate reversely (in an opposite direction) so that the exposed portion of the alignment layer 3 which is not protected by the photoresist pattern 7 is rubbed in a direction opposite to that of the other portion of the alignment layer 3 protected by the photoresist pattern 7. Thus, a plurality of areas of which alignments differ in directions respectively can be formed in each pixel using the property that the rubbing directions of the alignment layer are changed according to the rotating direction of the rubbing roll.
Moreover, the multi-domain structure, as shown in FIG. 2B, can be achieved by a photo-alignment method of irradiating ultraviolet light (UV-rays) using a mask 7a blocking an alignment layer selectively instead of using the rubbing roll.
Namely, after an alignment layer 3 is uniformly coated on a substrate 1, rubbing is carried out thereon in one direction, as shown in FIG. 2A, using a rotating rubbing roll 5 to form a first pre-tilt angle θ1.
Thereafter, UV-rays are irradiated over a blocking mask 7a, as illustrated in FIG. 2B, to form a second pre-tilt angle θ2 smaller than the first pre-tilt angle θ1 in the exposed alignment layer 3. The above-explained steps are repeated to form multiple domains.
FIG. 3A is a diagram illustrating a pair of domains in one pixel and their corresponding rubbing directions, and FIG. 3B is a cross-sectional view illustrating alignment of liquid crystals along a bisecting line I-I′ in FIG. 3A.
Referring to FIG. 3A, one pixel is divided into a first area A and a second area B. An upper substrate and a lower substrate are rubbed in directions opposite to each other, respectively, so that alignment directions of the upper and lower substrates are opposite to each other. In the drawing, a solid line indicates the rubbing direction of the upper substrate, and the dotted line indicates the rubbing direction of the lower substrate.
FIG. 3B is a cross-sectional view along a bisecting line I–I′ in FIG. 3A, in which a schematic feature of aligned liquid crystals is shown.
Referring to FIG. 3B, because an alignment layer 13a on an upper substrate 10a and an alignment layer 13b on a lower substrate 10b are rubbed in directions opposite to each other, respectively, the alignment direction of liquid crystals 15 differs in 180° direction for each of the upper and lower substrates 10a and 10b. Moreover, the alignment directions of the liquid crystals in the area A are symmetric to those in the area B, taking into account each alignment direction boundary, at which the alignment directions are reversed, of the upper and lower alignment layers 13a and 13b. Hence, it is possible to form the two domain, of which alignment directions of the liquid crystals are opposite to each other, in the areas A and B, respectively. In this case, light transmitted through the domain boundary where the alignment direction of the liquid crystals is reversed is represented as a disclination line in the form of a black or white line. Namely, the light cannot pass through the liquid crystals at the domain boundary, but instead only passes through the upper substrate. Hence, if polarized directions of upper and lower polarizing plates attached respectively to the upper and lower substrates are normal or, perpendicular to each other, a black line appears on a screen. If they are in parallel, a white line appears on the screen. However, when a voltage is applied to the common electrode of the upper substrate and the pixel electrode of the lower substrate, the disclination line appearing on the screen draws an ‘S’ type curve. Namely, for a liquid crystal display device having a domain boundary in the middle of a pixel, as shown in FIG. 4, directions of liquid crystals 30 arranged in a domain boundary area are respectively changed by a fringe field generated between a corresponding pixel 21b and a neighboring pixel 21a or 21c (i.e., the pixel at the left or right of the corresponding pixel 21b). Specifically, the boundary plane of the rearranged liquid crystals 30, i.e., a disclination line 20, makes an ‘S’ type curve. In this case, in order to block the display failure (white or black line), a black matrix 25 is formed on this area. However, since the disclination line 20 makes the ‘S’ type curve, the area of the black matrix 25 must be wide enough to cover the entire “S” shaped disclination. Hence, an aperture ratio is reduced.